This application claims the priority benefit of Taiwan application serial no. 90111679, filed on May 16, 2001.
1. Field of the Invention
The invention relates in general to a method of forming a micro-via. More particularly, the invention relates to a method of forming a micro-via that electrically connects neighbouring conductive layers in a printed circuit board.
2. Description of the Related Art
In the recent years, as the electronic products have been developed to be light, thin, short and small, the layout and fabrication of a printed circuit board (PCB) has necessarily been improved. To increase the layout density of the circuit, in addition to thinning the wires, a via used to electrically connect two neighboring conductive layers can be shrunk to operate the circuit with an enhanced efficiency.
In the prior art, a method of mechanically drilling through a hole is used to form a via hole in a printed circuit board. The minimum bore of the via hole is 0.2 mm. Being restricted by the accuracy of the mechanical process, the bore cannot be further decreased. Therefore, further wire refinement cannot be achieved. If a laser is used to for the hole drilling process, the bore can be further reduced.
As the integration of the electronic products increases, and the function thereof becomes more powerful the number of deposition layers is increased from one or two to five or six, and may even exceed ten. The electronic products can thus be accommodated in the printed circuit board with a more compact space. Accordingly, the bores of the laser drilled holes for high density interconnection becomes smaller and smaller. Once the position of the hole slightly shifts or deviates, the electric conduction is seriously affected. As a result, the reliability of the printed circuit board is lowered, and the fabrication cost is greatly increased due to the degradation of yield.
FIG. 1A to FIG. 1G show the method of forming a via for connecting the neighboring conductive layers in a printed circuit board using a laser. In FIG. 1A, a substrate 10 is provided. The substrate 10 can be an inner substrate or an insulation layer. A patterned conductive wiring layer 12 is formed on the substrate 10 by photolithography and etching. In FIG. 1B, an insulation layer 14 is formed on the conductive wiring layer 12 and the substrate 10. In FIG. 1C, a patterned conductive wiring layer 16 with openings 18 are formed on the insulation layer 14.
In FIG. 1D, in the direction indicated by the arrows 20, a laser drill operation is performed on the insulation layer 14 exposed by the openings 18. In FIG. 1E, via holes 22 are formed in the insulation layer 14 to expose a surface of the conductive wiring layer 12. In FIG. 1F, using screening printing, the via hole 22 is filled with a conductive material that serves as the vias 24 electrically connecting the conductive wiring layers 12 and 16. Or, as shown in FIG. 1G, a via copper layer 26 is plated onto surfaces of the via holes 24, followed by filling the via holes 24 with an insulation layer 28. The via copper layer 26 provides the electric connection between the conductive wiring layers 12 and 16.
The above laser drill through hole method introduces the following problems:
1. In the laser drilling process, if the laser is not precisely aligned with the opening of the conductive wiring layer, the insulation layer exposed by the opening cannot be effectively removed. Thus, conduction between two conductive wiring layers cannot be established, and the original wiring layout is altered.
2. The bore of the via can be reduced to less than 0.2 mm using a laser drilled through hole. However, the bore is still much larger than that of the via formed using photolithography, so there is still a limit to bore shrinkage when using the laser drill.
3. When filling the via hole with conductive material, if a cavity structure such as a bubble is formed in the conductive material, a popcorn structure is formed during heating. This damages the conductive wiring structure.
4. For the screen printing method, the conductive material filling the via hole uses many metal conductive particles mixed with liquid glue to increase fluidity. However, the material uniformity of the conductive material is consequently decreased to affect the stability of conduction.
The invention provides a method of forming a micro-via that provides electric connection between neighboring conductive wiring layers of a printed circuit board to resolve the problems occurring in the prior art. The method can be applied to fabrication and design of the layout of a circuit board. A substrate and a patterned conductive wiring layer on the substrate are provided. A copper layer is plated on surfaces of the substrate and the conductive wiring layer. A photoresist layer is formed on the plated copper layer. A part of the photoresist layer is removed to expose a part of the plated copper layer. Using the plated copper layer as a seed layer, a conductive pillar is formed on the exposed plated copper layer. The photoresist layer is removed, and the plated copper layer that is exposed after removing the photoresist layer. Another insulation layer is formed on surfaces of the substrate and the conductive pillars. A part of the insulation layer is removed to expose the conductive pillar. Another copper layer is plated onto surfaces of the conductive pillars and the insulation layer. A second photoresist layer is formed on the plated copper layer. A part of the second photoresist layer is removed to expose a part of the second copper layer. Again, using the second copper layer as a seed layer, a conductive pillar is formed on the exposed second copper layer. The second photoresist layer is removed.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.